Metals such as advanced superalloy materials have been used in a wide variety of applications including turbine engines, bearings, gears, etc. Frequently, the materials are constantly employed under severe environmental and mechanical conditions, resulting in an accelerated degradation of the performance of the materials. Corrosion and wear problems have been widely recognized as a main cause in reducing the lifetime of the material. Robust coatings which are thermally, chemically, and mechanically stable on metals have not been effectively provided heretofore.
Diamonds are known to have chemical inertness, extreme mechanical hardness, and the highest thermal conductivity of all solid materials. Currently, polycrystalline diamond thin films can be easily deposited using a chemical vapor deposition (CVD) technique. However, diamond film deposition on metal surfaces is not an easy, and sometimes not an appropriate task to do, for several reasons, which follow.
Differences in thermal properties between the substrate and the deposited film can introduce severe stress at the film substrate interface. The thermal expansion coefficients of most metals far exceed that of diamond over a wide range of temperatures, thus giving rise to spontaneous spallation failure between the film and the substrate. Polycrystalline diamond films by themselves are not suitable, particularly in the applications which involve heavy thermal cycling.
Metals such as iron, cobalt, nickel, or platinum are known for their catalytic effects. These elements have the tendency to form coke deposits from gases containing carbon at elevated temperatures. The coke deposits range in structure from polycyclic aromatic rings to disordered carbon, usually in the form of laminar graphite, nonoriented carbon, and fibrous carbon films. These undesired deposits significantly affect the nucleation of diamond particles on such metals. Thus far no satisfactory explanation has been given to account for the catalytic effect, although it has been speculated that the catalytic effect might be due to the high electron affinity which originates from an unfilled d-band electron shell in the metals. Moreover, the rate of carbon diffusion in metals such as iron is high enough that the carbon species from the diamond forming plasma environment are consumed significantly by the materials.
From the foregoing, it can be appreciated that there is a need to provide an improved diamond or diamond-like film for coating metals.